In wireless communication systems, mobility between different geographical areas is provided by allowing mobile stations to handoff their communications between base stations. The base station from which a mobile station is handing off from is typically referred to as a serving base station, and the base station to which the mobile station is to hand off to is typically referred to as a target base station. Mobile stations can include cellular telephones, personal digital assistants (PDAs), computers equipped with wireless transceivers (such as integrated transceivers or transceivers on PCMCIA cards), pagers, and the like.
There are two general types of handoff, hard handoff and soft handoff. Hard handoff is performed in a break-before-make manner, in which a mobile station ceases all traffic and control channel communications with the serving base station and then attempts to establish traffic and control channel communications with the target base station. This approach is used mostly to preserve air-link and network resources utilized by mobile stations in the handoff areas, and might be the preferred mode for high-speed packet data channels. It also simplifies the handling of data packets at the network infrastructure. However, hard handoff does not provide full diversity gain associated with soft handoff, thereby increasing the possibility of temporary session interruption. Such a temporary interruption would impact Quality of Service (QoS) for real-time applications, such as voice.
Using a soft handoff technique, a mobile station will simultaneously communicate with the serving and target base stations over both base stations traffic and control channels. This type of handoff provides diversity, as the mobile station can receive the same information from both base stations. Due to low chance of session interruption, soft handoff is typically preferred where real-time applications such as voice are involved. However, soft handoff requires synchronous resource allocation and scheduling, and coordinated packet handling on the network side. This has a direct impact on throughput efficiency and network complexity. Soft handoff also increases the required backhaul capacity, especially for high rate traffic channels. Additionally, it is only feasible when frequency reuse is 1:1 and strict traffic synchronization is required among all bases stations involved in the soft handoff. Moreover, soft handoff complicates packet scheduling and automatic retransmission request (ARQ) messages.
While many wireless communication technologies provide either soft or hard handoff, broadband communication technologies including cdma2000, 1xEV-DO, WCDMA, Flarion's Flash OFDM® and IEEE80216e provide both hard and soft handoff. Specifically, soft handoff is typically employed for communications that are sensitive to interruptions or errors, while hard handoff is employed for all other communications.
Exemplary embodiments of the present invention provide a hybrid handoff scheme in which control channel communications are handed off from a serving base station to a target base station, while the mobile station continues to perform data communications with the serving base station. Once the handoff of the control channel communications are completed, the data communications are handed off from the serving base station to the target base station.
In accordance with one embodiment of the present invention, the serving base station informs the mobile station of time periods, corresponding to frames, during which the mobile station can perform the signaling with the target base station necessary for handing off the control channel communications from the serving base station to the target base station. The serving base station then adjusts its scheduler in such a way that the serving base station transmits unicast and multicast messages to the mobile station during time periods other than those during which the mobile station is performing signaling with the target base station.
In accordance with another embodiment of the present invention, a fast cell switch feedback channel is allocated to the mobile station when there is a possibility or a need for a handoff. The mobile station uses the fast cell switch feedback channel to perform the signaling necessary for handing off the control channel communications from the serving base station to the target base station. The mobile station then can handoff the traffic channel communications from the serving base station to the target base station.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.